Apparatus for separating blanks from plastic sheets

ABSTRACT

Mirrors for use by dentists or for similar purposes are produced by clamping a blank of strongly light-reflecting synthetic thermoplastic material between annular male and female mirror frame portions. Such clamping step is carried out simultaneously with separation of the blank from a web of sheet material which is maintained in a flattened and stretched condition and is weakened in the zone around the blank by a welding electrode which softens the material of the web and penetrates into the thus softened material by being biased toward a supporting surface located opposite the electrode. The male mirror frame portion serves as a means for tearing the blank from the weakened zone and for simultaneously moving the marginal portion of the thus separated blank against the internal surface of the female mirror frame portion.

[ 1 July 25,1972

APPARATUS FOR SEPARATING BLANKS FROM PLASTIC SHEETS Inventors: GerhardZuhlke, Geroldswil; Paul Brandli,

Aurich; Max Spaltl, Zurich, all of Switzerland Assignee: Zuhlke &Brandli AG, Zurich, Switzerland Filed: March 16, 1971 App1.No.: 124,786

Related U.S. Application Data Division of Ser. No. 819,404, April 25,1969, Pat. No. 3,623,209.

Foreign Application Priority Data Aug. 14, 1968 Switzerland ..l2230/68Nov. 5, 1968 Switzerland ..16434/68 US. Cl ......29/33 K, 29/200 R,225/96,

156/257, 156/261, 156/510 Int. Cl. ..B23p 23/00, 823p 19/00 Field ofSearch ..29/200 R, 33 K, 33 S, 413,

[56] References Cited UNITED STATES PATENTS 3,331,726 7/1967 McGinley156/5 10 3,274,665 9/1966 l-ligginson et al.. ...29/33 K 2,982,4565/1961 l-lsu et al ..225/2 Primary Examiner-Gil WeidenfeldAttorney-Michael S. Striker [5 7] ABSTRACT Mirrors for use by dentistsor for similar purposes are produced by clamping a blank of stronglylight-reflecting synthetic thermoplastic material between annular maleand female mirror frame portions. Such clamping step is carried outsimultaneously with separation of the blank from a web of sheet materialwhich is maintained in a flattened and stretched condition and isweakened in the zone around the blank by a welding electrode whichsoftens the material of the web and penetrates into the thus softenedmaterial by being biased toward a supporting surface located oppositethe electrode. The male mirror frame portion serves as a means fortearing the blank from the weakened zone and for simultaneously movingthe marginal portion of the thus separated blank against the internalsurface of the female mirror frame portion.

Fig. 5

IN V EN TOR.

Gram 20001:: 0400 (war lllx PAM:

APPARATUS FOR SEPARATING BLANKS FROM PLASTIC SHEETS CROSS-REFERENCE TORELATED APPLICATION This application is a division of our co-pendingpatent application Ser. No. 819,404, filed Apr. 25, 1969 and entitled f-Method for Separating Blanks From Plastic Sheets" and now U.S. Pat. No.3,623,209.

BACKGROUND OF THE INVENTION The present invention relates to anapparatus for separating blanks from sheets or webs of flexiblematerial, particularly for separating substantially circular ordisk-shaped blanks from sheets of strongly light-reflecting syntheticthermoplastic material. Such blanks can be used in small mirrors,particularly in those which are employed by a dentist in the mouth of apatient.

It is already known to employ blanks of foil or plastic sheet stock inmirrors for use by dentists or for analogous purposes, especially as alight-reflecting means in relatively small mirrors which are mounted ina circular frame at one end of a handle so that the dentist or thephysician can readily manipulate the mirror during introduction of thereflecting surface into the mouth of a patient. As a rule, the blanksare separated from a relatively large sheet of light-reflecting materialby resorting to stamping or punching tools. The sheet is placed onto aflat elastic support and the sharp cutting edge of the punching orstamping tool is moved toward the support to thereby separate a blankfrom the flattened sheet. The useful life of the elastic support is veryshort and the cutting edges of tools must be sharpened or replaced atfrequent intervals. Furthermore, the freshly separated blank must bepicked up and inserted into a suitable holder for clamping between theportions of a mirror frame. Such positioning and orientation of theblank consumes much time and contributes significantly to the cost ofthe ultimate product.

It was already proposed to clamp the sheet in the region around thecutting edge while the stamping tool moves against the support or toclamp the blank within the confines of the cutting edge so that thefreshly separated blank can be more readily manipulated during insertionbetween the portionsof a mirror frame. A drawback of such procedure isthat the severing of blanks entails considerable losses in sheetmaterial and that the blank must be trimmed subsequent to clamping in amirror frame. The trimming operation must be performed by skilledpersons and by resorting to complicated and expensive equipment.

SUMMARY OF THE INVENTION An object of the invention is to provide anovel and improved apparatus for separating blanks from sheet material,particularly for separating relatively small blanks from lightreflectingsynthetic plastic sheet material, in such a way that the losses inmaterial are reduced to a minimum and that the separated blank is in anoptimum position for further processing.

A further object of the invention is to provide an apparatus constructedin such a way that the separation of successive blanks does not resultin dulling of cutting edges, in wear on movable or stationary partsand/or in improper orientation of freshly formed blanks.

An additional object of the invention is to provide an apparatus whereinthe separation of blanks from a sheet of thermoplastic material takesplace simultaneously with further processing of the blank, preferablysimultaneously with introduction of the blank between the portions of amirror frame.

Still another object of the invention is to provide an apparatus whichcan be used for automatic production of blanks in rapid sequence andwith a high degree of reproducibility so that the size and orientationof each of a series of successively obtained blanks is the same as thatof the preceding blank or blanks.

A concomitant object of the invention is to provide an apparatus whichcan produce small mirrors for use by dentists or for similar purposesand wherein the light-reflecting portions of the mirrors are formed,manipulated and installed in mirror frames in a series of rapidlyfollowing steps.

One feature of our invention resides in the provision of an apparatusfor separating blanks from sheets, particularly for separatingsubstantially disk-shaped (circular) blanks from sheets of stronglylight-reflecting synthetic thermoplastic material. The method practicedon this apparatus comprises the steps of flattening and stretching asheet whose dimensions exceed the dimensions of the desired blank (suchsheet may constitute a portion of a web which is being intermittentlywithdrawn from a reel or from an analogous source of convoluted sheetmaterial), weakening the thus flattened and tensioned sheet in a zonealong the outline of the desired blank, and tearing the part within theweakened zone away from the remainder of the sheet so that the thusseparated part constitutes a blank.

The weakening step preferably comprises heating the aforementioned zoneof the tensioned sheet, most preferably with simultaneous application ofpressure to reduce the thickness of the plasticized zone. Such heatingand pressure applying steps are preferably effected by resorting to asubstantially ring-shaped welding electrode. The method preferablyfurther comprises the step of cooling the weakened zone prior to tearingso that the zone adheres to the electrode during tearing. During suchtearing, the weakened zone is preferably mechanically clamped betweenthe electrode and the surface of a stationary support. The tearing steppreferably comprises separating the blank from the weakened zone inresponse to application of a uniform pressure to the part within theweakened zone in a direction at right angles to the plane of thetensioned sheet. Such tearing step is preferably carried outsimultaneously or substantially simultaneously with clamping of themarginal portion of the blank between two complementary annular surfaceswhich can be provided on two frame portions forming part of a dentistsmirror or the like.

The novel features which are considered as characteristic of theinvention are set forth in particular in the appended claims. Theimproved apparatus itself, however, both as to its construction and itsmode of operation, together with additional features and advantagesthereof, will be best understood upon perusal of the following detaileddescription of certain specific embodiments with reference to theaccompanying drawing.

BRIEF DESCRIPTION OF Til-IE DRAWING FIG. 1 is a schematic partlyelevational and partly sectional view of an apparatus which embodies oneform of the present invention;

FIG. 2 is an enlarged vertical sectional view of a detail in theapparatus of FIG. 1, showing that stage of a blank formation when theelectrode has completed the weakening of the zone around a blank;

FIG. 3 is an enlarged vertical sectional view of a detail of thestructure shown in FIG. 2, a freshly formed blank being indicated byphantom lines during the initial stage of separation from the remainderof a synthetic thermoplastic sheet;

FIG. 4 illustrates the final stage of separation of a freshly formedblank; and

FIG. 5 is a schematic view of the electric heating means for theelectrode in the apparatus of FIG. ll.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The apparatus which is shown inFIG. 1 comprises a housing or main support 1 which carries a source ofsupply of convoluted sheet material here shown as a reel 2 which isrotatable about a horizontal axis. The reel 2 can be rotated in adirection (arrow B) to pay out the web or sheet 40 by an electric motor3 which is mounted on the housing 1 and drives a pulley 2a on the coreof the reel 2 by way of an endless belt 4.

The sheet 40 consists of synthetic plastic material and is stronglylightreflecting so that its surfaces can be used as mirror surfaces. Thehousing 1 further supports a horizontal pivot 50 for an elongated lever5 which carries at its lower end a tensioning roll 6 for the sheet 40.The pivot 5a is located at a level above the axis of the reel 2 and thelever 5 is biased in a counterclockwise direction, as viewed in FIG. 1,by a helical spring 7 one end of which is secured to the housing 1. Thesheet 40 passes over a guide roll 8 and into the nip of twointermittently drive advancing rolls 9, 10. The path along which thesheet 40 moves between the guide roll 8 and advancing rolls 9, 10 ishorizontal and is parallel to the top surface 66 of a base plate lawhich forms part of the housing 1 and supports the shafts of theadvancing rolls 9, 10. The latter is rotatable stepwise in a clockwisedirection (arrow C) by a drive which includes a radial arm 11 and asingle-acting fluid operated cylinder and piston unit 12. Thearrangement is such that the arm 11 rotates the advancing roll 10 onlywhen it turns in a clockwise direction but not when it rotates in thecounterclockwise direction (under the action of the spring 12a in thecylinder 12b of the unit 12. The strokes of the unit 12 are selected insuch a way that the sheet 40 is advanced stepwise and always through adistance of predetermined length which suffices to permit the formationof a blank 68 (see FIGS. 24). The tensioning roll 6 and spring 7 insurethat the sheet 40 is flattened and is held under tension in the regionbetween the guide roll 8 and advancing rolls 9, 10.

The top surface 66 of the base plate 1a of the housing 1 is providedwith a recess for a holder or anvil 50 which carries the annular femaleportion 51 of a mirror frame. The positioning of the holder 50 is suchthat the upper edge face of the female frame portion 51 thereon islocated slightly below the level of and is parallel with the surface 66,i.e., parallel with the plane of that portion of the sheet 40 whichextends between the guide roll 8 and advancing rolls 9, 10.

The base plate 1a carries two upstanding vertical guide members or tierods 21 which are located at diametrically opposite sides of the holder50 and serve to guide a vertically reciprocable outer carrier or yoke22. The means for moving the yoke 22 toward and away from the holder 50comprises a link 23 which is connected to an eccentric pin 24 ofa disk25 driven by an electric motor 26 by way of an endless belt 27. Thecentral portion of the yoke 22 sup ports a downwardly extending secondholder or ram 28 which detachably supports an annular male mirror frameportion 52 in axial alignment with the female mirror frame portion 51 onthe lower holder 50. The tie rods 21 further guide a second orintermediate reciprocable carrier 53 having an external annular shoulder54 which is biased against a complementary internal shoulder of the yoke22 by helical springs 55 surrounding the tie rods in the space betweenthe parts 22, 53. The intermediate carrier 53 defines an internalchamber or bore 56 which accommodates an annular flange 61 of a third orinner carrier 57. Helical springs 58 in the bore 56 bias the carrier 57downwardly so that its external annular shoulder 59 normally abutsagainst a complementary internal shoulder of the intermediate carrier53. The lower end face of the inner carrier 57 is provided with anannular recess for a portion of a ringshaped weakening electrode 60which also serves as a means for facilitating separation of a blank 68from the sheet 40. The bias of the springs 58 is selected in such a waythat the pressure which the electrode 60 exerts upon the sheet 40 cannotexceed a predetermined value. The springs 55 insure that theintermediate carrier 53 shares the initial downward movement of the yoke22 and that the yoke can thereupon move with reference to the carrier 53in order to introduce the male mirror frame portion 52 into the femalemirror frame portion 51 and to simultaneously clamp a freshly formedblank 68 between the frame portions 51, 52. The annular flange 61 of theinner carrier 57 is formed with a convex external surface (seeparticularly FIG. 2) to insure that the carrier 57 can be tilted withreference to the carrier 53 so that the lower edge surface of theelectrode 60 can move into full surface-to-surface abutment with theadjoining ring-shaped zone 68a of the sheet 40 and presses such zone ofthe sheet against the top surface 66. In cooperation with the springs58, the flange 61 insures that each portion of the electrode 60 exertsupon the ad- 5 joining portions of the sheet 40 the same force when theyoke 22 is caused to perform a downward stroke.

FIG. 2 illustrates in greater detail a portion of the structure shown inFIG. 1 and during that stage of the formation of a blank 68 when theelectrode 60 dwells in its lower end position and penetrates into theplasticized material of the adjoining zone 68a of the sheet 40. Theelectrode 60 is heated, at least during that stage of a cycle when itengages the sheet 40, so that the zone 68a becomes plasticized andpermits penetration of the electrode into the thus plasticized material.A layer of insulating material (indicated by a legend) is providedbetween the electrode 60 and the adjoining portion of the inner carrier57; this layer insures satisfactory insulation up to a potential ofabout volts and at a temperature of up to 500C. The insulating layer ispreferably a good conductor of heat and can stand substantial mechanicalstresses. During penetration of the electrode 60 into the upper side ofthe sheet 40, the underside of the zone 68a abuts against the topsurface 66 of the base plate 1a.

FIG. 3 illustrates on a larger scale a portion of the structure shown inFIG. 2. The lower part of the electrode 60 has penetrated into theplasticized material of the zone 68a. The pressure which is transmittedto the electrode 60 by the yoke 22, carriers 53, 57 and springs 55, 58,combined with short lasting intensive heating of the electrode, insuresthat the lower part of the electrode penetrates well into theplasticized material of the zone 68a so that the remainder of the sheetis connected to the resulting disk-shaped blank 68 only by way of arather thin web (in zone 68a) disposed between the lower end surface ofthe electrode 60 and the top surface 66 of the base plate la. Thethickness of the web in the weakened zone 68a is indicated at C. Thewidth of the web in the zone 68a corresponds to the distance between theinternal and external surfaces 62, 63 of the lower part of the electrode60. The electrode 60 is preferably cooled, at least immediately after itreaches the lower end position shown in FIG. 3, whereby the plasticizedmaterial of the weakened zone 68a sets and adheres to the surfaces 62,63 as well as to the lower edge surface of the electrode. If desired,the inner carrier 57 can be provided with cooling ribs or fins toaccelerate the cooling ac tion; this inner carrier constitutes a meansfor cooling the electrode 60 when the latter has penetrated into thesheet 40. In the next stage of a cycle, the upper holder 28 is moveddownwardly with reference to the arrested inner carrier 57 and electrode60 whereby the female mirror frame portion 52 engages the disk-shapedblank 68 within the confines of the electrode 60 and causes the marginalportion of the blank to flex downwardly in a manner as shown in FIG. 3by broken lines. This causes the blank 68 to be torn away and to thusbecome separated from the internal annular surface 62 of the lower partof the electrode 60. The annular surface surrounding the marginalportion of the blank 68 is shown at 64. The numeral 67 denotes theinternal surface of the sheet 40 above the weakened zone 68a. Furtherdownward movement of the upper holder 28 causes the blank 68 to tear atthe lower end of the surface 62 so that it becomes separated from thezone 68a (see FIG. 4). The resulting initial gap is shown at 65. Suchseparation of the blank 68 from the remainder of the sheet 40 can becompleted by exertion of a relatively small force because the sheet isweakened in zone 68a. Furthermore, the weakened zone is clamped betweenthe lower edge surface of the electrode 60 and the top surface 66 sothat the separation of the blank 68 is predictable and is completedwithout the formation of irregularities along the lower part of theannular surface 64 of the blank 68. The electrode 60 continues to clampthe zone 68a to the top surface 66 of the base plate la during assemblyof the mirror, i.e., during introduction of the upper mirror frameportion 52 into the lower mirror frame portion 51. Thus, the sheet 40cannot interfere with introduction of frame portion 52 into the frameportion 51. The properly assembled portions 51, 52 then form an annularframe of a mirror which further includes the blank 68; the marginalportion of such blank is automatically clamped between the portions 51,52 when the upper holder 28 reaches the lower end of its stroke, namely,when the eccentric pin 24 on the disk 25 reaches its lower end position.

The operation of the means for moving the yoke 22 (motor 26) issynchronized with operation of the driving unit 12 for the advancingrolls 9, in such a way that the yoke 22 is held in upper end positionwhen the unit 12 advances the sheet 40 by a step so as to place a freshportion of the sheet 40 into the space between the holders 50, 28 whichrespectively support fresh mirror frame portions 51, 52. Such mirrorframe portions can be introduced into the apparatus by hand or by anautomatic or semiautomatic feeding assembly of any known design. It isto be noted that the apparatus further comprises a substantiallyring-shaped stripping or disengaging member 70 (FIG. 1) which causes thezone 68a and the adjoining portion of the sheet 40 to become separatedfrom the electrode 60 when the yoke 22 causes the intermediate carrier53 to lift the inner carrier 57 to the position shown in FIG. 1. Thisenables the rolls 9, 10 to advance the sheet 40 by a step when the parts22, 53, 57 assume the positions shown in FIG. 1. If desired, theelectrode 60 can heat and plasticize the adjoining portion of the sheet40 before the yoke 22 is moved upwardly so as to facilitate theseparating or disengaging action of the stripping means 70.

FIG. 5 illustrates an electrical heating device for the electrode 60.The circuit of this heating device comprises a transformer 73 whosesecondary winding 71 is connected with term'inals 72 of the electrode 60and whose primary winding 74 is in circuit with an energy source 71a(e.g. an outlet for 220 volt 50 kHz current). The primary winding 74 isin series with a suitable timer 75 which can complete the circuit of theprima ry winding at desired intervals and for desired periods of time toinsure satisfactory heating action. The electrode 60 constitutes aresistor which is heated in response to flow of current through thesecondary winding 71.

A feature of the improved apparatus is that the dimensions A, B (FIG. 2)can be kept to a minimum. This results in reduced waste in sheetmaterial and renders it unnecessary to subject the blank 68 to asecondary treatment (trimming) prior to assembly with the mirror frameportions 51, 52.

If desired, the lower axial end of the male frame portion 52 or 152 (asviewed in FIG. 4 or 5) can be provided with a bottom plate whichprevents penetration of dust, dirt or other foreign matter to the innerside of the respective blank 68. Such bottom plate also enhances theappearance of the mirror. The manner in which the mirror including theportions 151, 152 and blank 68 of FIG. 5 is separably connected to asuitable handle is preferably the same as shown in FIGS. 1 and 3.

The unit 12 is only responsible for advancing stepwise the sheet 40. Byadvancing the sheet 40, the lever 5 is advanced to its forward position(indicated by dotted lines) in which it contacts a switch (not shown)which starts the motor 3. The motor 3 turns the reel 2 until the lever 5reaches its rear position; the lever 5 then contacts a second switch(not shown) which interrupts the electric circuit of the motor 3.

The sheet 40 consists of polyester. Its thickness is preferably between5 and microns. If the sheet 40 is used for the production of dentalmirrors, a layer of aluminum is vaporized upon it.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featureswhich fairly constitute essential characteristics of the generic andspecific aspects of our contribution to the art and, therefore, suchadaptations should and are intended to be comprehended within themeaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent is:

1. In an apparatus for separating blanks from sheets, particularly forseparating substantially circular blanks from sheets of stronglylight-reflecting synthetic thermoplastic material, a combinationcomprising stretching means for tensioning and flattening a sheet whosedimensions exceed the dimensions of the desired blank; weakening meansfor weakening the thus tensioned sheet in a zone along the outline ofthe desired blank; separating means for separating the part within thethus weakened zone from the remainder of the sheet so that the thusseparated part forms a blank; and first and second holder means arrangedto support cooperating frame portions of a mirror at the opposite sidesof the tensioned sheet, one of said holder means constituting saidseparating means and being arranged to separate the blank from saidweakened zone by way of the respective frame portion and tosimultaneously move such frame portion into engagement with the otherframe portion to thereby clamp the separated blank between the frameportions.

2. A combination as defined in claim 1, wherein said weakening meanscomprises a heatablle electrode having a surface whose configurationcorresponds to the configuration of said zone and which is adjacent toone side of the tensioned sheet, support means having a surface adjacentto the other side of the tensioned sheet, and means for moving one ofsaid surfaces toward the other surface whereby the electrode softens thematerial of said zone and said surface thereof is caused to penetrateinto the thus softened material.

3. A combination as defined in claim 2, further comprising means forintermittently heating said electrode and means for cooling theelectrode at least upon penetration into the material of the sheet sothat the weakened zone adheres to the electrode during separation of theblank.

4. A combination as defined in claim 1, further comprising a source ofconvoluted sheet, said stretching means comprising a pair of advancingrolls engaging the sheet from opposite sides and drive means forintermittently rotating at least one of said rolls in a direction toeffect withdrawal of a predetermined length of sheet material from saidsource, said weakening means being located between said source and saidadvancing rolls.

5 A combination as defined in claim 4, wherein said stretching meansfurther comprises a guide member located between said source and saidweakening means, said sheet being trained over said guide member.

6. A combination as defined in claim 5, wherein said stretching meansfurther comprises a spring-biased tensioning device engaging the sheetbetween said guide member and said source.

7. A combination as defined in claim 1, wherein said weakening meanscomprises a substantially annular electrode and electric heating meansfor said electrode.

8. A combination as defined in claim 7, wherein said electrode is a ringof fiat metallic material and constitutes an electric resistance in thecircuit of said electric heating means.

9. A combination as defined in claim 7, wherein said weakening meansfurther comprises a support having a surface adjacent to one side of thetensioned sheet opposite said electrode, and further comprising carriermeans movable toward and away from said surface and supporting saidelectrode, and electric insulator means interposed between said carriermeans and said electrode.

10. A combination as defined in claim 9, wherein said carrier meansconstitutes a device for cooling said electrode.

11. A combination as defined in claim 7, wherein said heating meanscomprises timer means for timing the length of intervals during whichsaid electrode is heated and the length of periods between successiveintervals.

12. A combination as defined in claim 1, wherein said weakening meanscomprises a support having a surface adjacent to one side of thetensioned sheet, an electrode adjacent to the other side of the sheetopposite said surface, means for heating said electrode, and] firstcarrier means for moving said electrode toward and away from saidsurface so that the electrode heats and weakens said zone of the tensioned sheet when moved toward said surface, and further comprisingadditional carrier means for moving said first carrier means toward saidsurface and resilient means interposed between said first and saidadditional carrier means.

13. A combination as defined in claim 12, wherein said electrode istiltable with reference to said additional carrier means.

14. A combination as defined in claim 12, further comprisweakening meanscomprises an electrode adjacent to one side of the tensioned sheet, asupport having a surface adjacent to the other side of the tensionedsheet opposite said electrode, and means for reciprocating saidelectrode toward and away from said surface, said separating meanscomprising a separating member operatively connected with saidreciprocating means.

17. A combination as defined in claim 16, wherein said reciprocatingmeans comprises a first carrier connected with said electrode, a secondcarrier movable with and with reference to said first carrier andarranged to move said first carrier and said electrode with reference tosaid surface, and drive means for moving said second carrier, saidsecond carrier constituting a yoke and said separating member beingconnected with said yoke.

1. In an apparatus for separating blanks from sheets, particularly forseparating substantially circular blanks from sheets of stronglylight-reflecting synthetic thermoplastic material, a combinationcomprising stretching means for tensioning and flattening a sheet whosedimensions exceed the dimensions of the desired blank; weakening meansfor weakening the thus tensioned sheet in a zone along the outline ofthe desired blank; separating means for separating the part within thethus weakened zone from the remainder of the sheet so that the thusseparated part forms a blank; and first and second holder means arrangedto support cooperating frame portions of a mirror at the opposite sidesof the tensioned sheet, one of said holder means constituting saidseparating means and being arranged to separate the blank from saidweakened zone by way of the respective frame portion and tosimultaneously move such frame portion into engagement with the otherframe portion to thereby clamp the separated blank between the frameportions.
 2. A combination as defined in claim 1, wherein said weakeningmeans comprises a heatable electrode having a surface whoseconfiguration corresponds to the configuration of said zone and which isadjacent to one side of the tensioned sheet, support means having asurface adjacent to the other side of the tensioned sheet, and means formoving one of said surfaceS toward the other surface whereby theelectrode softens the material of said zone and said surface thereof iscaused to penetrate into the thus softened material.
 3. A combination asdefined in claim 2, further comprising means for intermittently heatingsaid electrode and means for cooling the electrode at least uponpenetration into the material of the sheet so that the weakened zoneadheres to the electrode during separation of the blank.
 4. Acombination as defined in claim 1, further comprising a source ofconvoluted sheet, said stretching means comprising a pair of advancingrolls engaging the sheet from opposite sides and drive means forintermittently rotating at least one of said rolls in a direction toeffect withdrawal of a predetermined length of sheet material from saidsource, said weakening means being located between said source and saidadvancing rolls. 5 A combination as defined in claim 4, wherein saidstretching means further comprises a guide member located between saidsource and said weakening means, said sheet being trained over saidguide member.
 6. A combination as defined in claim 5, wherein saidstretching means further comprises a spring-biased tensioning deviceengaging the sheet between said guide member and said source.
 7. Acombination as defined in claim 1, wherein said weakening meanscomprises a substantially annular electrode and electric heating meansfor said electrode.
 8. A combination as defined in claim 7, wherein saidelectrode is a ring of flat metallic material and constitutes anelectric resistance in the circuit of said electric heating means.
 9. Acombination as defined in claim 7, wherein said weakening means furthercomprises a support having a surface adjacent to one side of thetensioned sheet opposite said electrode, and further comprising carriermeans movable toward and away from said surface and supporting saidelectrode, and electric insulator means interposed between said carriermeans and said electrode.
 10. A combination as defined in claim 9,wherein said carrier means constitutes a device for cooling saidelectrode.
 11. A combination as defined in claim 7, wherein said heatingmeans comprises timer means for timing the length of intervals duringwhich said electrode is heated and the length of periods betweensuccessive intervals.
 12. A combination as defined in claim 1, whereinsaid weakening means comprises a support having a surface adjacent toone side of the tensioned sheet, an electrode adjacent to the other sideof the sheet opposite said surface, means for heating said electrode,and first carrier means for moving said electrode toward and away fromsaid surface so that the electrode heats and weakens said zone of thetensioned sheet when moved toward said surface, and further comprisingadditional carrier means for moving said first carrier means toward saidsurface and resilient means interposed between said first and saidadditional carrier means.
 13. A combination as defined in claim 12,wherein said electrode is tiltable with reference to said additionalcarrier means.
 14. A combination as defined in claim 12, furthercomprising guide means for reciprocably guiding said carrier means formovement substantially at right angles to said surface and means forreciprocating said additional carrier means including a yokereciprocable along said guide means and resilient means interposedbetween said yoke and said carrier means.
 15. A combination as definedin claim 14, further comprising drive means for reciprocating said yokewith reference to said guide means.
 16. A combination as defined inclaim 1, wherein said weakening means comprises an electrode adjacent toone side of the tensioned sheet, a support having a surface adjacent tothe other side of the tensioned sheet opposite said electrode, and meansfor reciprocating said electrode toward and away from said surface, saidseparating means comprising a separating member operatively connectedwith said reciprocating means.
 17. A combination as defined in claim 16,wherein said reciprocating means comprises a first carrier connectedwith said electrode, a second carrier movable with and with reference tosaid first carrier and arranged to move said first carrier and saidelectrode with reference to said surface, and drive means for movingsaid second carrier, said second carrier constituting a yoke and saidseparating member being connected with said yoke.